Laminar flow electron gun and method

ABSTRACT

A laminar flow electron gun for forming an electron beam including a cathode for emitting electrons, an apertured dishshaped electrode surrounding the cathode surface and an anode spaced from said cathode and electrode and cooperating therewith to provide a substantially uniform electric field at the surface of said cathode to cause electrons to emit normally from the entire surface in a beam, said anode also forming a divergent electrostatic lens along the path of the beam and accelerating and focusing means disposed further along the path of the beam to accelerate and focus the beam at a target.

United States Patent Silzars et a1.

LAMINAR FLOW ELECTRON GUN AND METHOD Inventors: Aris Silzars, RedwoodCity; David J.

Bates, Los Altos, both of Calif.

Assignee: Watkins-Johnson Company, Palo Alto, Calif.

Filed: June 3, 1971 Appl. No.: 149,445

References Cited UNITED STATES PATENTS 1/1952 Dodds 11/1959 Currie315/31 R 315/31 R Spangenberg 315/14 1 June 19, 1973 2,902,523 9/1959Knechtli 315/15 2,935,636 5/1960 Knechtli..... 315/15 X 2,888,606 5/1959Beam 315/16 [57] ABSTRACT A laminar flow electron gun for forming anelectron beam including a cathode for emitting electrons, an apertureddish-shaped electrode surrounding the cathode surface and an anodespaced from said cathode and electrode and cooperating therewith toprovide a substantially uniform electric field at the surface of saidcathode to cause electrons to emit normally from the entire surface in abeam, said anode also forming a divergent electrostatic lens along thepath of the beam and accelerating and focusing means disposed furtheralong the path of the beam to accelerate and focus the beam at a target.

24 Claims, 9 Drawing Figures Patented June 19, 1973 3,740,607

4 Sheets-Sheet 1 F I g. l

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Cathode Magnification Current Efficiency INVENTOR. Ari; Silzars Dawd J.Bates 2 a W, m

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Ans SIIZGI'S LL BY DGVId J. Bares Attorneys 1 LAMINAR FLOW ELECTRON GUNAND METHOD GOVERNMENT RIGHTS The invention herein described was made inthe course of or under a contract with the Department of the Navy.

BACKGROUND OF THE INVENTION This invention relates generally to anelectron gun and more particularly to a laminar flow electron gun whichprovides a small electron beam with high current density and whichrequires minimum power for beam generation, modulation and deflectionwith minimum gun length.

Presently, electron guns for display tubes or long beam lengths in afield-free region are of the crossover type. This type of gun isschematically illustrated in FIG. 1. It consists of two basic sections,one, a beam forming section frequently called the triode section and,second, a focusing section which focuses the beam eitherelectrostatically or electromagnetically.

The key elements of the triode section are the cathode, the grid and theaccelerating electrode. The cathode serves as the electron source and isusually an indirectly heated plane surface. The grid or modulatingelectrode is usually a cup with a perforated bottom. Typically, theaperture area is much less than the cathode surface area. The firstanode or accelerator electrode is usually a cylinder with a limitingaperture. Operation of crossover guns have been treated in detail byseveral authors. One description of operation is given by I. G. Maloffand E. W. Epstein in Electron Optics in Television", Mcgraw-Hill BookCompany, 1938.

The crossover gun has certain inherent defects and limitations. Anon-uniform cathode loading due to variations in the magnitude of theelectric fields across the surface of the cathode. Non-uniform cathodeloading means that the cathode has to be run hotter than in a gun withuniform cathode loading and results in shorter operating life.Non-uniform loading also forms a focused spot having non-uniformbrightness distribution.

In the crossover gun there are substantial changes in spot size withvariations in grid drive. As a result, the resolution of the gun is bestat low beam currents (low brightness in the case of a CRT) and degradesas the beam current (brightness) increases. The changes in spot sizewith grid drive result from the geometry of the triode section of thecrossover gun which is such that changes in grid potential not onlyalter the emitted current density but also the size of the emittingarea.

Physically long gun lengths are necessary to minimize beammagnification. Apertures inserted to improve resolution result ininefficient current utilization because of beam interception. It hasbeen shown, based on the consideration of the optics of thermallyemitted electrons, that the maximum current density, with perfectfocusing, at crossover of the cathode image is given by 1/1 l/M [l (1,-M sin d2) exp (Ve/kT) M sin M2 where J is the current density at thefocused spot,J., is the cathode current density, M is the geometricalmagnification (the ratio of the crossover or cathode-image diameter tocathode diameter which is proportional to the image distance divided bythe object distance) I is the half angle of the beam envelope measuredat the target including all electron paths reaching the point inquestion, T is the cathode temperature in degrees Kelvin, k is theBoltzman constant and V is the potential at the point in question.

Limiting values are of interest. For M large (magnification),

J,,./J l/IW and for M small (demagnification) where 1,, is the largestpossible value of current density that can be achieved under anycondition. The intensity efficiency J/J measures how well any gunperforms as a function of the magnification.

A curve of the J/J,, available at the screen for various values of M andd: is shown in FIG. 2 for the case of (eV/kT) 10,000 (this correspondsto a voltage of about 800 volts since e/V has a value of 11,000 and T isabout l,000K. for an oxide cathode). 05 in this case is the valuedetermined by a limiting aperture.

Examination of FIG. 2 indicates that as the magnification decreases, theintensity efficiency increases. To achieve a small spot size, themagnification should be less than unity. As the intensity efficiencyincreases, the amount of cathode current that reaches the screendecreases, the balance being intercepted by limiting apertures. Thefraction of cathode current used can be called the current efficiencywhich is:

Current efficiency (JM lJ FIG. 3 is a plot of the intensity efficiencyversus the current efficiency. These curves show that to approach thelimiting value of intensity efficiency, most of the current must bewasted.

In most applications of the crossover gun, the requirement for minimumtube depth severely limits the performance of the gun so that theintensity efficiency is far from the maximum. This can be understood byconsidering that the object focused on the viewing screen is locatednear the cathode, the distance from the focus lens being a matter ofafew inches. The image is formed several or more inches from the focuslens at the screen. Thus, the ratio of the image to object distance issubstantially greater than unity which is contrary to achieving highresolution. In those cases where high resolution is essential, the tubebecomes quite lengthy to reduce the magnification. Thus, a typicalmicrospot tube with a 5 inch screen diameter is 25 inches long.

Finally, some crossover guns have a high sensitivity to dimensionaltolerances because of the short focal length lenses and limitingapertures used. The aberrations present in the relatively short focallength triode section cause transverse velocities to be introduced inthe beam trajectories. These transverse velocities result in beamspreading. Even if the cathode current density were uniform, the spotsize at a given focal point would increase.

Another type of electron gun which has found wide application inelectron tubes such as travelling wave tubes and klystrons is anelectron gun employing a Pierce Electrode. The cathode in this type ofgun operates with high current density and high efficiency. This type ofcathode or gun employs a cathode surrounded by a closely spaceddish-shaped electrode at zero potential and an anode spaced from andconfronting the cathode. The beam is substantially at its final velocityOBJECTS AND SUMMARY OF THE INVENTION It is a general object of thepresent invention to provide an improved electron gun for use in cathoderay tubes, camera tubes, storage tubes, electron bombarded semiconductordevices and other devices utilizing electron beams.

It is a further object of the present invention to provide an electrongun of the above type which may be unmodulated, intensity modulated, ordeflection modulated.

It is another object of the present invention to provide an improvedelectron optics system for electron guns.

It is a further object of the present invention to provide an electrongun which projects a high current density beam of circular or othercross-section and which requires no focusing of the electron beam in thefieldfree region beyond the gun and which will focus a beam at a pointwhose distance beyond the gun is in the order of one hundred times ormore the beam diameter at the focus point.

It is another object of the present invention to provide an electron gunwith uniform current density distribution across the beam.

It is a further object of the present invention to provide a laminarflow electron gun which can be made smaller in length and diameter thanexisting crossover guns.

It is another object of the present invention to provide an electron gunwhich will project a relatively small constant spot size with variationsin grid drive.

It is another object of the present invention to provide an electron gunwhich has improved efficiency, that is, a gun in which essentially theentire cathode current arrives at the target and all of the cathodesurface is used for emission.

It is a further object of the present invention to provide an electrongun in which the electric field lines at the cathode are uniform andnormal to produce a uniform loading to thereby reduce the peak currentdensity at the cathode surface for a given total beam current and toprovide a beam having no transverse velocities other than thermalvelocities.

It is another object of the invention to provide an electron gun inwhich small grid voltages are required to control the beam current.

The foregoing and other objects of the invention are achieved by anelectron gun which is adapted to provide a laminar flow electron beamcomprising a cathode for providing electrons, an apertured dish-shapedcontrol electrode surrounding said cathode surface and providing acontinuation thereof, a cylindrical anode spaced along the axis fromsaid cathode, one end of said anode being shaped to cooperate with thecontrol electrode to provide a substantially uniform electric field atand adjacent to the cathode surface thereby to provide substantiallyuniform current emission from the surface of the cathode and laminarflow and a field forming an electrostatic lens spaced from the cathodealong the path of the beam and additional electrode means disposed alongthe path of the beam for receiving the beam leaving said anode andfocusing and accelerating the beam.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a schematic diagram of aprior art crossover electron gun.

FIG. 2 is a graph showing the intensity efficiency as a function ofmagnification for prior art guns of the type shown in FIG. 1.

FIG. 3 is a graph showing the density efficiency as a function ofcurrent efficiency for prior art guns'of the type shown in FIG. 1.

FIG. 4 is a schematic diagram of a cathode ray tube including a laminarflow electron gun in accordance with the invention.

FIG. 5 is an enlarged view of the electron gun of FIG. 4.

FIG. 6 is a view of the gun of FIG. 5 showing the equipotential linesand the electron beam.

FIG. 7 shows a gun in accordance with the present invention includingmagnetic focusing.

FIG. 8 is a graph showing cathode current as a function of grid voltagefor a gun in accordance with the present invention.

FIG. 9 is a graph showing the spot size as a function of beam currentfor a gun in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 is a schematic drawing ofa crossover electron gun in accordance with the prior art. It includestriode section 11 having a heated cathode 12 with emitting surface 13.An apertured cup-shaped control electrode 14 is spaced in front of thecathode surface. A first anode 16 accelerates the electrons and formsthe last element of the triode section 11. The anode 16 includes currentlimiting apertures 17 and 18. The focusing section includes a second orfocusing electrode 19 spaced along the path of the beam. The anodeincludes a current limiting aperture 21. The action of the triodesection is to focus the electrons leaving the surface at a point 22where the electrons spread and are intercepted by the limiting apertures17, 18 and 21 and focused to impinge upon a target 23. As is well known,the focusing section may include magnetic means rather thanelectrostatic means. Deflection means are not shown.

The grid voltage e,,, accelerating voltage e, and focusing voltage e,are applied to the electrodes. The defects and limitations of this typeof gun were previously described. FIGS. 2 and 3 show the intensityefficiency as a function of current efficiency for a crossover gun ofthe type illustrated in FIG. 1 and described.

FIGS. 4, 5 and 6 show a cathode ray tube 31 and an electron gun 32 inaccordance with the invention. The improved electron gun may also beused in camera tubes, storage tubes, beam semiconductor devices and inother applications where a high efficiency, high current, sharplyfocused electron beam is required.

The gun includes an indirectly heated cathode 33 heated by a resistiveheater 36 disposed in the cupshaped rear portion of the cathode. A smallcathode emitting surface 35 is disposed at the end of projection 36 todefine an area of predetermined size. An electrode 37 including aperture38 surrounds and is spaced from the cathode projection 36. Thedish-shaped surface 39 of electrode 37 is adjacent to and cooperateswith the cathode surface 35. An anode 41 is placed in front of thecathode surface 35 and electrode surface 39. The anode 41 may be acylinder including a rim or lip 42. The rim or lip 42 cooperates withthe cathode and electrode to provide a substantially uniform electricfield 43, FIG. 6, across the emitting surface 35 of the cathode wherebyelectrons are uniformly emitted substantially normal to the surface ofthe cathode to form a laminar beam. The electrode 41 also forms anelectrostatic lens as indicated by the field lines 44 whereby the beamleaving the cathode is defocused. In accordance with the invention, thelens is divergent whereby the beam 46 is expanding as it travels intothe focusing section. In the present example a second anode 47cooperates with the first to form a convergent lens which converges thebeam field lines 48. The beam then travels into the region includingconductive surface 49 in the inside of the cathode ray tube. This is inessence a third electrode which provides the final accelerating field,the final converging lens 50 and a fieldfree region for the beam to flowand focus on the screen 51 of the cathode ray tube.

Once the electron beam leaves the final anode, it is not under theinfluence of any focusing forces. There are, however, effects which tendto spread or defocus the beam such as space charge repulsion forces,transverse thermal velocities and transverse velocities due toaberrations and/0r gun assymetries. Typically, the beam diameter has tobe increased by the divergent lens of anode 41 from its originaldiameter (equal to that of the cathode), so that it can subsequently befocused by the focusing fields onto the target or screen at adiameterthe same or less than that of the cathode. It is to be notedthat in contrast to a Pierce Gun, the beam does not have its finalvelocity until it leaves the gun assembly. As a result, it is possibleto control the emission from the cathode (beam current).

The cathode 33 is supported at one end of refractory cylinder 55 whichacts as a heat shield. The other end of the cylinder is supported as asupport 52 which has support discs 53. Spaced ceramic pins 54 aresupported by the tube envelope and extend through the discs to supportthe cathode. The electrode 37, anode 41 and anode 47 include discs 56,57 and 58 which are also engaged by the ceramic pins which support thegun assembly in the envelope with the various members in alignment.

Referring to FIG. 5, the length, diameter and spacing of the variouselectrodes is shown as well as the applied voltages. The grid voltagee,,, the anode voltage e focusing electrode voltage e and acceleratingvoltage 2 are shown applied between the cathode 33 and grid 37, anode41, focusing electrode 47 and accelerating electrode 48, respectively.The slope of the dish-shaped electrode is represented by (#1 and theslope of the lip or rim 42 of anode 41 is represented by (12 bothmeasured from a line perpendicular to the axis of the tube. The spacingbetween the bottom of the grid 37 adjacent to the opening 38 and thecathode surface is represented by the distance a The spacing between thebottom of the grid 37 and the end of the lip 42 is represented by thedistance d The spacing between the anode 4i and the electrode 47 isrepresented by the distance d;,. The diameter of the cathode isrepresented by the diameter D, and the opening 35 of the dishshaped gridby the diameter D The diameter of the opening in the lip 42 isrepresented by the diameter D;, and the diameter of the cylindricalportion of the anode 41 and electrode 47 by the diameter D,. Thediameter of the final accelerating electrode which may comprise theconductive coating in the inside of the cathode ray tube is representedby the diameter D The length of the anode 41 and the electrode 47 isrepresented by I, and 1 respectively. The overall length of the gunbeyond the end of the cathode surface is represented by the length 1 andthe length of the field-free space is represented by the length 1,.

The electron-optical design for an electron gun of the type described isas follows. Referring to FIG. 5, the angles Q51 and 42 and the distanced, and the cathode curvature are selected so that the electrons leavethe sur face of the cathode in substantially parallel paths (laminarflow) normal to the surface. The voltages and shapes are selectedwhereby the equipotentials are substantially parallel, and the gradientor field is substantially perpendicular to the cathode surface in thevicinity of the surface and the fields are such as to form a diverginglens adjacent the aperture in the lip 42. This is accomplished byselecting the distances, angles, voltages and diameter D of the lip 42to control the angle at which the beam is launched from the cathode. Thebeam 46 is schematically illustrated in FIG. 6 and is a beam which isessentially perpendicular to the cathode. The positive focusing actionof the fields 48 and and the field-free region within the electrode 49provide an image at the screen. It is apparent to one skilled in the artthat the electrostatic focusing may be aided by the electromagneticfocusing structure 60, as schematically shown in FIG. 7, which serves toform a converging lens to converge and focus the beam at the screen.

The size of the spot at the screen will be determined by the positionand size of the virtual cathode image which serves as the object for theconverging lens. Trajectories are such that ideally an infinitely smallcathode image can be produced at any desired position behind thecathode. This is indicated by the dotted line 61 projected behind thecathode in FIG. 6. In practice, however, the size of the virtual cathodeimage will be limited by transverse thermal velocities and variousimperfections such as spherical aberrations and astigmatism. However,the laminar flow gun minimizes these limitations in three ways: thecathode virtual image is produced much further from the focal point ofthe converging lens than in a crossover gun; thermal velocity effectsare reduced by uniformly accelerating the beam to its final velocity orvoltage with electrodes 41, 47 and 49 in a comparatively short distance;and aberration effects are reduced by using long focal length lenseswithout limiting apertures.

The virtual cathode position and size are of primary importance sincethe converging lenses magnify this virtual cathode in direct proportionto the distances of the object image from the lens focal points. Incomparison, a crossover gun, by design, must also produce a cathodeimage near the cathode. This causes excessive magnification of thecrossover spot and, therefore, when high resolution devices arerequired, they must be made very long with much of the beam interceptedby the apertures in the focusing electrodes.

The focus point and the cathode emitting area are relatively unaffectedby changes in the voltage v, on the control electrode 37. Furthermore,relatively low voltages are required to make substantial changes in thebeam current. The primary effect of changing the grid voltage in thelaminar flow gun is to uniformly change the cathode current density andnot the cathode emitting area. Thus, the uniformity of current densityand focus spot positions are substantially less affected by changes ingrid drive used to change the total beam current. If the beam current isextremely small, such as used in existing cathode ray tubes, the beam inthe gun region need not significantly be expanded, if at all, and yetthe beam can be focused on the screen by the converging lens to form anextremely small, high current density spot.

The cathode image in the laminar flow gun of the present invention isideal because of the uniform current density emission from the cathodeand the parallel electron beam in the cathode-anode region. The opticalanalog is that the beam originates from a point source an infinitedistance away so that the illumination is perfectly uniform and parallelat the cathode. The combined result of this invention is a much moreuniform high current density beam at the screen. This is achieved withimproved cathode life and a much smaller overall gun length. Aspreviously described, the gun design beyond the anode is conventionalsince it basically consists of two cylindrical electron focusing lenses.A single electrostatic lens or a magnetic lens could also be used. Theonly requirement is that the virtual cathode image produced in thecathode-anode region be imaged on the screen with minimum aberration orimage distortion. it is, of course, apparent that deflection means,either magnetic or electrostatic, may cooperate with the beam after itleaves the gun to control its deflection or position at the target. Suchmeans were not shown to simplify the disclosure. They are well known inthe art.

A cathode ray tube incorporating an electron gun in accordance with theinvention was constructed and it had the following dimensions:

d 0.012 inches d 0.065 inches d 0.035 inches 1 0.188 inches 1 0.320inches 1 0.620 inches 1 3.50 inches D 0.028 inches D 0.060 inches D0.040 inches D 0.190 inches D 0.300 inches 4n The applied voltages wereas follows:

a, (grid) 0 volts (normal full on) e 480 volts e, 1,700 volts e 12,500volts The grid cut-off characteristic for an electron gun in ode currentis controlled over a range from 0 to 400 microamps. It is also to beobserved that substantially the total cathode current reaches the screenthereby providing essentially percent cathode efficiency. In FIG. 9 acurve showing the line size as a function of beam current is set forthfor the above gun. The line width was obtained in a tube in which thedeflection of the beam was 2,000 inches per second at a 60 Hertzrepetition rate. It is to be noted that with a change in beam current offour times, the spot size only changes about 46 per cent.

By way of example, another cathode ray tube having an electron gun inaccordance with the invention was constructed and operated as follows:

d 0.006 inches d 0.150 inches d 0.030 inches 1 0.200 inches 1 1.24inches 1 1.626 inches 1 3.50 inches D 0.010 inches D 0.018 inches D0.160 inches D 0.375 inches D 0.750 inches in With the grid voltage egat -l0 volts, the anode voltage 2 at 480 volts and the voltage e at1,700 volts, the, beam was very highly convergent in the region of theanode aperture 42 and the line width 0014-00 1 5 inches andsubstantially independent of the screen voltage e This indicates thatthe beam size is aberration limited. With the grid voltage eg at 0volts, the anode voltage e, at volts and the voltage e at 1,700 volts,the line width was 0.006 inches for e at 12,500 volts and varied between0.0095 and 0.0055 with screen voltages between 6,500 and 14,500respectively. This indicates that the beam size is Langmuir limited.

It is apparent from the above description that the gun may be used inconnection with other charged particles to accelerate them from a sourceand project and focus a beam at a screen or target.

We claim:

1. A particle gun for providing a beam of charged particles comprising asource of particles, an apertured dish-shaped control electrodesurrounding said source and providing substantially a continuationthereof, an electrode spaced along the axis from said source and beingat a potential with respect to the source and control electrode whichwill accelerate particles from the source, said electrode beingshaped'to cooperate with the control electrode and source to provide asubstantially uniform electric field at the source surface for initiallyaccelerating the charged particles to substantially the same velocity toprovide a substantially perpendicular uniform laminar flow of particlesat the source surface and also providing an electric field spaced fromthe source along the path of the beam defining an electrostatic lens andat least an additional electrode along the path of the beam at a higherpotential than said electrode for further accelerating the particle beamleaving said first electrode and focusing it on a target.

2. A particle gun as in claim 1 wherein said source is a cathode forproviding electrons.

3. A particle gun as in claim 1 wherein said electrode spaced along theaxis from said source includes an aperture for passing the beam, thediameter of said aperture being substantially larger than the diameterof the beam.

4. A particle gun as in claim 1 wherein means providing a magneticfocusing field cooperates with said additional electrode means.

5. A particle gun as in claim 1 wherein the angle of the surface of saiddish-shaped electrode with respect to a plane perpendicular to the axisof the gun is between and 45.

6. An electron gun for providing an electron beam comprising a cathodehaving a surface providing a source of electrons, an apertured controlelectrode surrounding said cathode and having a surface providingsubstantially a continuation of the cathode surface, an anode spacedfrom said cathode surface and control electrode and being at a morepositive potential than said cathode and control electrode to cooperatetherewith to accelerate electrons at said surface to substantially thesame velocity to provide a substantially uniform laminar flow ofelectrons in a beam substantially perpendicular at said cathode surfaceand toward said anode, said anode providing a diverging field along thepath of the beam, and additional electrode means at a more positivepotential for receiving and accelerating said beam and also serving tofocus said beam.

7. An electron gun as in claim 6 wherein said anode means includes ancylindrical portion with an apertured-shaped end facing said cathode andcontrol electrode.

8. An electron gun as in claim 7 wherein the diameter of said anodeaperture is substantially larger than the diameter of the beam.

9. An electron gun as in claim 6 wherein the surface of said aperturedcontrol electrode defines an angle with a plane perpendicular to theaxis of the tube which angle is between 0 and 45.

10. An electron gun as in claim 7 wherein said additional electrodemeans comprises a cylindrical electrode for accelerating said beam andmeans providing final acceleration to said beam, said cylindricalelectrode and said means providing final acceleration forming a focusingfield.

11. An electron gun as in claim 10 wherein said last named means alsoprovides a field-free region along the path of said beam after it leavesthe cylindrical electrode.

12. An electron gun as in claim 10 wherein said cylindrical anode andcylindrical electrode are circular cylinders.

13. An electron gun as in claim 6 including means providing a magneticfocusing field cooperating with said additional electrode means.

14. A cathode ray tube having an envelope adapted to accommodate anelectron gun at one end and having 4 anode surface being shaped toprovide a substantially uniform electric field at said cathode surfaceto cause a laminar flow of electrons in a beam substantiallyperpendicular to said cathode surface at substantially the same velocitytoward said anode, and additional means for receiving and acting on saidbeam after it leaves the anode to focus said beam.

15. A cathode ray tube as in claim 14 wherein said anode means includesa cylindrical portion with an apertured-shaped end facing said cathodeand control electrode.

16. A cathode ray tube as in claim 15 wherein said additional electrodemeans comprises a cylindrical electrode further accelerating said beamand wherein the interior of the cathode ray tube envelope is conductivewhereby to provide an additional electrode for giving the beam its finalacceleration and cooperating with said electrode to focus the beam onthe target.

17. An electron gun as in claim 14 wherein the surface of said aperturedcontrol electrode defines an angle with a plane perpendicular to theaxis of the tube which angle is between 0 and 45.

18. An electron gun as in claim 15 wherein the diameter of said anodeaperture is substantially larger than the diameter of the beam.

19. The method of forming a small uniform electron beam with highcurrent density comprising the steps of generating an electric fieldnear a source of electrons which is substantially perpendicular to thesurface of the source to provide substantially parallel flow ofelectrons from the surface to form a laminar flow beam, providing adivergent electric field spaced from the source for receiving the beamand defocusing the beam after it has travelled a short distance from thesurface, providing at least one additional electric field along the pathof the beam for further accelerating said beam, said additional fieldalso providing a convergent field for focusing the beam.

20. The method of forming a small, high current density uniform electronbeam in a cathode ray tube of the type having a conductive coatedenvelope, an electron gun at one end of said envelope and a screen atthe other end which comprises the steps of providing an electric fieldwhich is substantially perpendicular to the surface of a source ofelectrons to provide substantially parallel flow of electrons from thesurface at substantially uniform velocity to form a laminar flow beam,providing an accelerating and focusing field along the path of the beamto further accelerate the beam and focus the beam, and providing anadditional final accelerating field between the gun and the coatedsurface of said envelope to give the beam its final acceleration andfocus the beam at the screen.

21. An electron gun for providing an electron beam comprising a cathodehaving a surface providing a source of electrons, a control electrodeadjacent said cathode surface and having a surface providing acontinuation of the cathode surface, an anode spaced from said cathodesurface and control electrode and cooperating therewith to causeelectrons to emit substantially normal from said surface and provide asubstantially uniform laminar flow of electrons in a beam from saidcathode surface towards said anode, and additional electrode means forreceiving and accelerating and focusing the beam.

22. An electron gun for providing an electron beam comprising a cathodehaving a surface providing a source of electrons, a control electrodeadjacent said cathode surface and having a surface providing acontinuation of the cathode surface, said control electrode being at avoltage substantially equal to or more negative than said cathode, ananode spaced from said cathode surface and control electrode, said anodeincluding a surface portion facing said cathode and control electrode,said anode being at a potential which is positive with respect to saidcathode and control electrode to cooperate therewith to cause electronsto emit substantially normal and at substantially the same velocity fromsaid cathode surface and provide a substantially uniform laminar flow ofelectrons in a beam from said cathode surface towards said anode, andadditional electrode means at a potential position with respect to saidanode for receiving and accelerating and focusing the beam.

23. An electron gun for projecting an electron beam comprising a cathodehaving a surface providing a source of electrons, a control electrodeadjacent said cathode and having a surface providing a continuation ofsaid cathode surface, said control electrode being at a potential equalto or more negative than said cathode, an anode spaced from said cathodesurface and said control electrode including a surface portion whichfaces said cathode surface and control electrode surface, said anodebeing at a potential which is positive with respect to said cathode andcontrol electrode, said control electrode and anode being shaped wherebyto cooperate and provide a substantially uniform accelerating electricfield at said cathode surface for initially accelerating electrons atsaid cathode surface toward said anode to substantially the samevelocity in a direction substantially perpendicular to said cathodesurface and also providing an electric field in the region between saidsurfaces which defines an electrostatic lens and at least one additionalelectrode means along the path of the electron beam at a potential morepositive than said anode to provide final acceleration to said beam.

24. A cathode ray tube as in claim 14 including means for applying acontrol voltage between said cathode and control electrode to controlthe beam current.

1. A particle gun for providing a beam of charged particles comprising asource of particles, an apertured dish-shaped control electrodesurrounding said source and providing substantially a continuationthereof, an electrode spaced along the axis from said source and beingat a potential with respect to the source and control electrode whichwill accelerate particles from the source, said electrode being shapEdto cooperate with the control electrode and source to provide asubstantially uniform electric field at the source surface for initiallyaccelerating the charged particles to substantially the same velocity toprovide a substantially perpendicular uniform laminar flow of particlesat the source surface and also providing an electric field spaced fromthe source along the path of the beam defining an electrostatic lens andat least an additional electrode along the path of the beam at a higherpotential than said electrode for further accelerating the particle beamleaving said first electrode and focusing it on a target.
 2. A particlegun as in claim 1 wherein said source is a cathode for providingelectrons.
 3. A particle gun as in claim 1 wherein said electrode spacedalong the axis from said source includes an aperture for passing thebeam, the diameter of said aperture being substantially larger than thediameter of the beam.
 4. A particle gun as in claim 1 wherein meansproviding a magnetic focusing field cooperates with said additionalelectrode means.
 5. A particle gun as in claim 1 wherein the angle ofthe surface of said dish-shaped electrode with respect to a planeperpendicular to the axis of the gun is between 0* and 45* .
 6. Anelectron gun for providing an electron beam comprising a cathode havinga surface providing a source of electrons, an apertured controlelectrode surrounding said cathode and having a surface providingsubstantially a continuation of the cathode surface, an anode spacedfrom said cathode surface and control electrode and being at a morepositive potential than said cathode and control electrode to cooperatetherewith to accelerate electrons at said surface to substantially thesame velocity to provide a substantially uniform laminar flow ofelectrons in a beam substantially perpendicular at said cathode surfaceand toward said anode, said anode providing a diverging field along thepath of the beam, and additional electrode means at a more positivepotential for receiving and accelerating said beam and also serving tofocus said beam.
 7. An electron gun as in claim 6 wherein said anodemeans includes an cylindrical portion with an apertured-shaped endfacing said cathode and control electrode.
 8. An electron gun as inclaim 7 wherein the diameter of said anode aperture is substantiallylarger than the diameter of the beam.
 9. An electron gun as in claim 6wherein the surface of said apertured control electrode defines an anglewith a plane perpendicular to the axis of the tube which angle isbetween 0* and 45* .
 10. An electron gun as in claim 7 wherein saidadditional electrode means comprises a cylindrical electrode foraccelerating said beam and means providing final acceleration to saidbeam, said cylindrical electrode and said means providing finalacceleration forming a focusing field.
 11. An electron gun as in claim10 wherein said last named means also provides a field-free region alongthe path of said beam after it leaves the cylindrical electrode.
 12. Anelectron gun as in claim 10 wherein said cylindrical anode andcylindrical electrode are circular cylinders.
 13. An electron gun as inclaim 6 including means providing a magnetic focusing field cooperatingwith said additional electrode means.
 14. A cathode ray tube having anenvelope adapted to accommodate an electron gun at one end and having ascreen at the other end including an electron gun comprising a cathodehaving a surface providing a source of electrons, an apertured controlelectrode surrounding said cathode surface and having a surfaceproviding substantially a continuation of the cathode surface, an anodespaced from said cathode surface and control electrode having a surfaceportion facing and cooperating with said cathode surface and controlelectrode surface, said anode being at a more positive potential thansaid cathode and control electrode, said anode surface being shaped toprovide a subStantially uniform electric field at said cathode surfaceto cause a laminar flow of electrons in a beam substantiallyperpendicular to said cathode surface at substantially the same velocitytoward said anode, and additional means for receiving and acting on saidbeam after it leaves the anode to focus said beam.
 15. A cathode raytube as in claim 14 wherein said anode means includes a cylindricalportion with an apertured-shaped end facing said cathode and controlelectrode.
 16. A cathode ray tube as in claim 15 wherein said additionalelectrode means comprises a cylindrical electrode further acceleratingsaid beam and wherein the interior of the cathode ray tube envelope isconductive whereby to provide an additional electrode for giving thebeam its final acceleration and cooperating with said electrode to focusthe beam on the target.
 17. An electron gun as in claim 14 wherein thesurface of said apertured control electrode defines an angle with aplane perpendicular to the axis of the tube which angle is between 0*and 45* .
 18. An electron gun as in claim 15 wherein the diameter ofsaid anode aperture is substantially larger than the diameter of thebeam.
 19. The method of forming a small uniform electron beam with highcurrent density comprising the steps of generating an electric fieldnear a source of electrons which is substantially perpendicular to thesurface of the source to provide substantially parallel flow ofelectrons from the surface to form a laminar flow beam, providing adivergent electric field spaced from the source for receiving the beamand defocusing the beam after it has travelled a short distance from thesurface, providing at least one additional electric field along the pathof the beam for further accelerating said beam, said additional fieldalso providing a convergent field for focusing the beam.
 20. The methodof forming a small, high current density uniform electron beam in acathode ray tube of the type having a conductive coated envelope, anelectron gun at one end of said envelope and a screen at the other endwhich comprises the steps of providing an electric field which issubstantially perpendicular to the surface of a source of electrons toprovide substantially parallel flow of electrons from the surface atsubstantially uniform velocity to form a laminar flow beam, providing anaccelerating and focusing field along the path of the beam to furtheraccelerate the beam and focus the beam, and providing an additionalfinal accelerating field between the gun and the coated surface of saidenvelope to give the beam its final acceleration and focus the beam atthe screen.
 21. An electron gun for providing an electron beamcomprising a cathode having a surface providing a source of electrons, acontrol electrode adjacent said cathode surface and having a surfaceproviding a continuation of the cathode surface, an anode spaced fromsaid cathode surface and control electrode and cooperating therewith tocause electrons to emit substantially normal from said surface andprovide a substantially uniform laminar flow of electrons in a beam fromsaid cathode surface towards said anode, and additional electrode meansfor receiving and accelerating and focusing the beam.
 22. An electrongun for providing an electron beam comprising a cathode having a surfaceproviding a source of electrons, a control electrode adjacent saidcathode surface and having a surface providing a continuation of thecathode surface, said control electrode being at a voltage substantiallyequal to or more negative than said cathode, an anode spaced from saidcathode surface and control electrode, said anode including a surfaceportion facing said cathode and control electrode, said anode being at apotential which is positive with respect to said cathode and controlelectrode to cooperate therewith to cause electrons to emitsubstantially normal and at substantially the same velocity from saidcathode surface and provide a sUbstantially uniform laminar flow ofelectrons in a beam from said cathode surface towards said anode, andadditional electrode means at a potential position with respect to saidanode for receiving and accelerating and focusing the beam.
 23. Anelectron gun for projecting an electron beam comprising a cathode havinga surface providing a source of electrons, a control electrode adjacentsaid cathode and having a surface providing a continuation of saidcathode surface, said control electrode being at a potential equal to ormore negative than said cathode, an anode spaced from said cathodesurface and said control electrode including a surface portion whichfaces said cathode surface and control electrode surface, said anodebeing at a potential which is positive with respect to said cathode andcontrol electrode, said control electrode and anode being shaped wherebyto cooperate and provide a substantially uniform accelerating electricfield at said cathode surface for initially accelerating electrons atsaid cathode surface toward said anode to substantially the samevelocity in a direction substantially perpendicular to said cathodesurface and also providing an electric field in the region between saidsurfaces which defines an electrostatic lens and at least one additionalelectrode means along the path of the electron beam at a potential morepositive than said anode to provide final acceleration to said beam. 24.A cathode ray tube as in claim 14 including means for applying a controlvoltage between said cathode and control electrode to control the beamcurrent.